Casting Impressions
Whether it's tool marks, foot prints, or tire tracks, casting is a simple tool for preserving impressions for later comparisons.  A simple kit would contain...

5 lbs. Plaster of Paris

2 sizes of Mixing Bowls

Reenforcement mesh to stabilize the cast

1 gallon distilled water

Identification tags with string

Gallon baggies (sometimes with Plaster premeasured for quick use)

Kits may also contain Dental Stone, which is thicker than Plaster, and Snow spray, which is similar to hair spray and hardens impressions in the snow before the casting is done.